Piedmont primrose-willow

Isnardia arcuata (Walter) Kuntze; I. pedunculosa (Michaux) de Candolle; Ludwigia pedunculosa Michaux; Ludwigiantha arcuata (Walter) Small

Herbs usually creeping and rooting at nodes, forming mats. Stems prostrate or decumbent and ascending at tips, slightly ridged, often well branched, 5–70 cm, glabrate to sparsely strigillose, denser on distal parts. Leaves opposite; stipules narrowly deltate or ovate, 0.05–0.15 × 0.05–0.1 mm; submerged stems: petiole 0–0.2 cm, blade narrowly linear, 1.9–4 ×0.1–0.25 cm; emergent stems: petiole 0–0.2 cm, blade narrowly elliptic or narrowly oblanceolate-elliptic to linear, 0.6–1.8 × 0.2–0.5 cm, base narrowly cuneate, margins entire, apex acute, surfaces glabrous or sparingly strigillose on margins and abaxial midveins; bracts reduced. Inflorescences in racemes or spikes, well-formed on ascending stems, not on prostrate stems; bracteoles attached at base of ovary or 1.5–8 mm proximally on pedicels, sublinear to very narrowly elliptic, 1.4–5 × 0.2–0.8 mm, apex acute, surfaces minutely strigillose. Flowers: sepals reflexed or spreading, green, lanceolate-deltate, 5.2–10 ×1.5–2.7 mm, with 3 prominent parallel veins, margins entire and minutely strigillose, apex acute or elongate-acuminate, surfaces minutely strigillose abaxially; petals rarely caducous, elliptic-obovate to spatulate-obovate, 7–11 × 4.5–8 mm, base attenuate, apex rounded; filaments initially spreading, becoming erect, yellow, 2.5–4.5 mm, anthers 1.3–2 × 0.7–1.1 mm; pollen shed in tight tetrads; ovary cylindric to funnelform, 4–5.5 × 1.5–2.8 mm; nectary disc elevated 0.6–1 mm on ovary apex, 1.5–2.6 mm diam., bright yellow, with 4 distinct domed lobes, minutely strigillose between lobes or glabrous; style yellow, 2.3–4(–4.8) mm, glabrous, stigma yellow, broadly capitate, 0.3–0.6 × 0.6–1.8 mm, as long as or exserted beyond anthers. Capsules clavate, subterete, sometimes slightly curved, 5.5–10 × 2.3–4 mm, hard-walled, irregularly dehiscent, pedicel (12–)17–45 mm. Seeds light to dark brown, elliptic-oblong, 0.5–0.7 × 0.3–0.4 mm, surface cells transversely elongate. 2n = 32.

Flowering Mar–Aug. Roadside ditches, edges of lakes or ponds, swampy prairies, springs, mucky or sandy beach strands; 0–150 m; Ala., Fla., Ga., S.C.

Ludwigia arcuata is common in its range, but geographically restricted to central and western parts of peninsular Florida and adjacent Georgia, extending to southern South Carolina. Disjunct populations have been collected in Bibb County in central Georgia and Mobile County, Alabama.

The tetraploid Ludwigia arcuata has the largest flowers in sect. Isnardia and is the most consistently outcrossing species; C. I. Peng (1989) reported abun­dant insect visitors on this species. It is morphologically most similar to the hexaploid L. brevipes, with which it shares two genomes (Peng).